Carbon dioxide sequestration and removing CO2 from the atmosphere or from gaseous effluents produced by electric power generation or other industrial activity, as examples, have become major environmental issues relating to global warming. Currently, membrane technology and chemistry related carbon dioxide removal processes are in use; however, the chemical systems used generate their own environmental issues.
The development of an efficient process for carbon dioxide removal from a gas mixture containing this molecule without the use of chemicals or other disposables would provide a method for controlling or reducing greenhouse gas emission, and assist in the process of sequestration of carbon dioxide. A similar problem exists for removing carbon monoxide from fuel cells, thereby reducing the carbon monoxide poisoning problem therein.
In a continuous medium (e.g., gas), sound is propagated as a wave from the source. The sound waves are transmitted by alternating compressions and rarefactions in adjacent gas layers. In a real fluid, sound is conducted by establishing an oscillating motion of discrete neighboring gas atoms and/or molecules and the gas is regarded as a continuum. The variations of gas density due to pressure changes in the gas layers induce an organized vibratory motion of the gas molecules. Therefore, fluctuations in sound pressure cause gas-borne particles to vibrate and possibly to collide. If the sound wave is contained in a resonant cavity and the frequency of the sound is such that standing waves of sound are established in the gas inside the cavity, one observes the establishment of pressure nodes and antinodes. Consequently, there are pressure gradients in the cavity that generate an acoustic radiation force that acts on air-borne particles (e.g., aerosols) and push these particles to the nodes or the antinodes depending on their size. This is the basis of the acoustic concentration of particles in acoustic standing wave.
U.S. Pat. No. 6,467,350 for Cylindrical Acoustic Levitator/Concentrator” which issued to Gregory Kaduchak and Dipen N. Sinha on Oct. 22, 2002, describes a low-power, acoustic apparatus for concentrating aerosols and small liquid/solid samples having particulates up to several millimeters in diameter. A commercially available, hollow cylindrical piezoelectric crystal which has been modified to tune the resonance frequency of the breathing mode resonance of the crystal to that of the interior cavity of the cylinder is used to establish a standing-wave pattern in the cavity which is effective for concentrating aerosol particles in air. Tuning the resonance frequency of the cylinder may be accomplished by removing an axial slice from the cylinder wall, choosing the composition of the piezoelectric cylinder, and by inserting a rod having appropriate dimensions into the cylinder, as examples. Accurate alignment of the resonant cavity is not required. Aerosol particles are directed to the resonance nodes that are in the form of rings; however, this makes actual collection of the aerosol particles difficult.
U.S. Pat. No. 6,644,118 for “Cylindrical Acoustic Levitator/Concentrator Having Non-Circular Cross-Section,” which issued to Gregory Kaduchak and Dipen N. Sinha on Nov. 11, 2003, describes the deformation of the circular cross-section of the transducer to concentrate the acoustic force along axial regions parallel to the axis of the transducer in order to obviate the need for a free-standing insert along the central axis of the cylinder to achieve this purpose, while causing particles in the fluid to concentrate within the regions of acoustic force for separation from a carrier fluid. In “Acoustic Concentration Of Particles In Piezoelectric Tubes: Theoretical Modeling Of The Effect Of Cavity Shape And Symmetry Breaking” by Shulim Kogan et al., J. Acoust. Soc. Am. 116 (2004) 1967-1974, provides a theoretical analysis of how the breaking of the spatial symmetry in cylindrical acoustic concentrators decreases the spatial distribution of the concentrated particles, thereby increasing the concentration efficiency and results in a simplification of the particle collection apparatus. The modified cavity is slightly elliptical in shape, which allows the nodal (and antinodal) rings to collapse into points. In the case of a generally cylindrical geometry with an elliptical cross-section, one can generate several lines parallel to the axis of the cylinder that represent the nodes where the aerosol particles collect. This makes the collection of the concentrated particles simpler. A few collection tubes may be placed at the appropriate locations near the end of the cylindrical cavity. It should be pointed out that the piezoelectric tube does not need to be elliptical in cross-section. Rather, the air cavity may be made elliptical by inserting two solid inserts having suitable curvature on each side on diagonally opposite ends of the circular cross-section of the piezoelectric tube that make good physical contact with the inner surface of thereof.
In “Trapping Of Heavy Gases In Stationary Ultrasonic Fields” by Rudolf Tuckermann et al., Chem. Phys. Letts. 363 (2002) 349-354, gases, such as CO2, were found to gather in stable rotational ellipsoidal systems around the pressure nodes of a stationary ultrasonic field. The authors state that: “With decreasing mass density of the sample gas the effect is reduced and totally disappears when sample gases with a lower mass density than the host medium are used . . . , there is strong evidence that the difference in mass density but not in temperature of the sample gas and the host medium are governing to effect.”, and “Assuming two nonmiscible fluids with different speeds of sound and with a plain common phase boundary a vertically propagating ultrasonic wave pushes the fluid with the lower speed of sound through the boundary layer into the other fluid independently of the direction of the wave. Transforming this example to two gases with different densities in a SUSF (stationary ultrasonic field) one can expect a similar result: according to Eq. (7), the gas with the higher mass density has a lower speed of sound and is sucked into the SUSF, then displaces the gas with the lower mass density and forms zones of trapped gas.”
In “Adsorption Effects In Light-Induced Drift” by G. Nienhuis, Optics Communications 62 (15 Apr. 1987) 81-85, light-induced drift resulting from excitation of optically active atoms immersed in a buffer gas is described. The effect arises from a difference in thermalization rate of the velocity distribution for both states of the atoms, due to different cross sections for elastic collisions with buffer-gas particles. For steady state conditions, a flow of excited atoms is not counterbalanced by an opposite flow of ground-state atoms, and a net flow of atoms remains leading to a density gradient in a closed cell. That is, light passing through an optically thick medium can move the atoms towards the dark end of the cell while leaving the buffer-gas particles untouched.